In the present invention, the term "trifluoromethylbenzene" includes benzene compounds that have at least one and possibly more trifluoromethyl-substituents.
One prior art process for the preparation of derivatives of benzene containing at least one trifluoromethyl group is described in U.S. Pat. No. 3,966,832. In accordance with that process, an autoclave is precharged with hydrofluoric acid alone or in homogeneous admixture with trichloromethylbenzene and hydrochloric acid to a pressure of at least 20 atmospheres (all pressures herein are absolute) in order to minimize the danger of an explosion, caused by too rapid an increase in pressure, upon starting the reaction. Trichloromethylbenzene and hydrofluoric acid are then introduced continuously into the autoclave under a pressure of more than 20 atmospheres; the hydrofluoric acid being in an excess of the required stoichiometric amount by at least 25 mole percent. After the reaction mixture has reached a given height, the liquid mixture is either withdrawn from the autoclave at a rate corresponding to the rate of feed, or else the mixture is passed into one or more additional autoclaves and withdrawn from the last autoclave of the cascade series. The initial autoclave and any autoclaves in the cascade are maintained at a temperature of between 80.degree. C. and 150.degree. C. throughout the process.
In order to obtain a sufficiently homogeneous mixture, as is necessary for proper operation of a continuous process, the reaction must usually be conducted at an autoclave pressure of 30 to 50 atmospheres. It is plain that operation under such pressures, particularly in an autoclave cascade or series arrangement, requires complex and costly equipment and procedures. Moreover, applicant has found that the required autoclave pressure is even further increased by the number of substituents present on the benzene ring of the trichloromethylbenzene starting material. This increase in required autoclave pressure is particularly important in the preparation of the various herbicides that comprise trifluoromethylbenzene having one or more chloro- substituents on the benzene ring. Therefore, in those important industrial processes the above-mentioned expense and complexity of equipment are magnified.
Another disadvantage inherent in this prior art process is the large excess of hydrofluoric acid which must be used in the process to achieve high product yield. As a result, the product mixture contains a large amount of unreacted hydrofluoric acid. This remaining acid must therefore be removed from the product and for any economic operation of the process be recycled to the feed of the initial autoclave. Again, these purification and recycle processes and procedures disadvantage the prior art process in time, in economics and in size and complexity of equipment required for its operation.